With the recent remarkable development and improvement of fluorine type ion-exchange membranes, electrolysis of sodium chloride solutions using an ion-exchange membrane as a diaphragm has become widespread. This technique is a method for producing hydrogen gas and sodium hydroxide in a cathode chamber and chlorine gas in an anode chamber by electrolysis of brine.
To reduce energy consumption, the use of a gas electrode as a cathode to conduct electrolysis while supplying oxygen to a cathode chamber to suppress hydrogen evolution and to greatly reduce cell voltage has been proposed, e.g., in JP-A-52-124496 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-B-2-29757 (the term "JP-B" as used herein means an "examined published Japanese patent application"), and JP-A-62-93388. In theory, cell voltage can be reduced by 1.2 V or more by converting a cathodic reaction with no supply of oxygen as represented by formula (1) to a reaction with oxygen supply as represented by formula (2): EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- (E=-0.83 V vs. NHE)(1) EQU H.sub.2 O+1/2O.sub.2 +2e.sup.- .fwdarw.2OH.sup.- (E=0.40 V vs. NHE)(2)
In general, a gas electrode is placed in a cathode chamber to partition the chamber into a solution chamber on an ion-exchange membrane side and a gas chamber on the opposing side. The gas electrode is usually prepared by molding a mixture of a hydrophobic substance, such as a polytetrafluoroethylene resin (hereinafter abbreviated as PTFE), and a catalyst or a catalyst-on-carrier, and its hydrophobic property hinders permeation of the liquid. However, the gas electrode gradually loses its hydrophobic property when exposed to a high temperature of around 90.degree. C. and a sodium hydroxide aqueous solution in a high concentration of about 32% by weight during long-term electrolysis. As a result, the liquid of the solution chamber begins to leak into the gas chamber. Furthermore, since the gas electrode is made of a mixture mainly comprising a carbon material and a resin, the gas electrode is mechanically brittle and tends to crack. These disadvantages have prevented practical use of a gas electrode for brine electrolysis.
FIG. 1 illustrates an electrolytic cell using a conventional gas electrode. Electrolytic cell 1 is partitioned by ion-exchange membrane 2 into anode chamber 3 and cathode chamber 4. Porous anode 5 is set close to ion-exchange membrane 2 in anode chamber 3. Anode chamber 3 has inlet 6 for feeding brine (a saturated sodium chloride aqueous solution) on the lower side wall thereof, outlet 7 for withdrawing a dilute salt water on the upper side wall thereof, and outlet 8 for withdrawing chlorine gas on the top thereof.
Cathode chamber 4 is equipped with gas electrode 11 comprising sheet substrate 9 having formed thereon electrode substance 10 comprising a mixture of a carbon material and PTFE, in such manner that cathode chamber 4 is partitioned into solution chamber 12 on the side of electrode substance 10 and gas chamber 13 on the side of substrate 9. Solution chamber 12 has inlet 14 for feeding a dilute aqueous solution of sodium hydroxide at the bottom thereof and outlet 15 for withdrawing a saturated sodium hydroxide aqueous solution at the top thereof. Gas chamber 13 has inlet 16 for feeding an oxygen-containing gas on the side wall thereof and outlet 17 for discharging an oxygen-containing gas at the bottom thereof.
Electolysis using a cell of this type is carried out by feeding brine to anode chamber 3 from inlet 6, a dilute sodium hydroxide aqueous solution to solution chamber 12 from inlet 14, and an oxygen-containing gas, such as air, to gas chamber 13 from inlet 16. Meanwhile, gas electrode 11, comprising sheet substrate 9 having thereon a layer of electrode substance 10, is damaged by the high temperature of the electrolytic solution and the concentrated sodium hydroxide aqueous solution generated by electrolysis. As a result, substance 9 and electrode substance 10 are deteriorated and fail to withstand long-term operation.
In order to solve the above-mentioned problem association with this type of an electrolytic cell, it has been proposed to unite a gas electrode with an ion-exchange membrane into an integral structure (hereinafter referred to as an integral gas electrode/ion-exchange resin type cell) without partitioning the cathode chamber, as disclosed in JP-B-61-6155. According to this method, the gas electrode reinforced by the ion-exchange membrane is said to overcome the mechanical brittleness. However, highly concentrated sodium hydroxide which is produced on the surface of the cathode, i,e., in the vicinity or on the surface of the ion-exchange membrane, penetrates the ion-exchange membrane and enters the anode chamber. This results not only in a reduction in current efficiency for sodium hydroxide production, but also presents the possibility of damage to the member constituting the anode chamber which usually has no alkali resistance. On the other hand, the sodium hydroxide produced on the surface of the ion-exchange membrane must permeate the gas electrode to allow for its recovery. However, it is extremely difficult to recover the sodium hydroxide while also feeding an oxygen-containing gas in sufficient amount to the gas chamber.